The subject invention relates to photoresist formulations for use in making printed circuit boards, lithographic plates, relief images and for various other uses in the graphic arts. More specifically, it deals with photoresist formulations with improved adhesion properties in both solvent and aqueous developable film so as to resist chemical attack and other kinds of degradation during etching, plating or other processing in any known manner.
A widely used technique in the manufacture of photoresists consists of the following steps: (1) a photopolymerizable composition is laminated on to a composite of a copper foil heat-set on a phenolic or epoxy resin board; (2) the laminate is then exposed imagewise to actinic light through a transparency in contact with a test pattern; (3) after exposure, the unexposed, pliable portions of the polymerizable composition are removed or washed away with a suitable solvent, thus exposing the underlying copper layer; and (4) the resulting copper layer is etched away for a sufficiently long period of time to form the desired depth of etch in the laminate; alternatively, the patterned metal surface is plated with a metal, such as copper, nickel, tin, chromium, silver or gold which is deposited in areas unprotected by the polymerized resist. When a sufficient amount of the metal has been deposited, the laminate is then treated to permit stripping of the resist mask from the laminate, thus providing a part having an etched or plated configuration corresponding to the pattern of the template or the negative through which the film was initially exposed.
It is especially important that the photoresist formulations employed to protect certain areas of the laminate to be plated or etched be chemically inert and resistant to corrosive etching or plating solutions, in order to obtain plated or etched configurations having sharp line definition and exhibiting a minimum amount of undercutting of the surface adjacent to the masked edges. It is equally important that the resist compositions exhibit good adhesion to the copper laminate, especially during the plating or etching operation, to prevent ingress of the etchant or plating solution between the resist and the substrate and in order to avoid any etching, printing or plating of the area or the part beneath the resist which it masks. Another criterion that must be met in a good photoresist material is that it can be readily stripped or removed from the surface subsequent to the plating or etching operation.
There has been a great deal of research activity devoted to the development of chemically inert photoresist compositions which exhibit the above-described properties, with little success and most of the formulations currently available fall far short of the requirements for exposure to certain types of plating or etching baths. Most often, the corrosive etching or plating solutions employed seriously affect the photoresist such that the entire resist layer or parts thereof peel off or are otherwise destroyed; or in the plating process, the electrolytically or electroless plated metal becomes detached from the laminate surface and gets washed away; the desired printed pattern thus becomes pitted, marred, discolored or haloed. Most of the prior art processes require the employment of a persulfate etch following the etching or electroless plating process, to remove the photoresist mask. This treatment not only removes the photoresist film but also dissolves some of the deposited copper. Sufficient allowance must therefore, be made for the loss of copper, so that the desired depth of copper plating is obtained. Furthermore, this persulfate treatment also results in pitting and marring of the metal surface thus making it unacceptable for printed circuit boards or lithographic plates or the like.
The prior art photopolymerizable compositions often contain small amounts of a dye which serves two purposes: (1) the dye imparts a color to the composition which is retained in the resist image and thus facilitates visibility and inspection for imperfections of the image on the printing board prior to the plating or etching process; (2) in many instances, the presence of the dye measurably increases the light sensitivity of the composition.
Prior art compositions formulated with certain substituted imidazoles and some benzotriazoles to somewhat improve the bonding qualities of photoresists, but these compounds also suffer from certain deficiencies. There is some peeling and blistering of the laminate observed if not subjected to persulfate treatment. The dry, unexposed photoresist films exhibit a relatively low shelf life under ambient conditions. The solvent of choice is also critical in that with some solvents, any solvent that gets entrapped in the film causes weathering problems in the dry unexposed photoresist films.
In recent times, however, some new benzotriazole derivatives have been synthesized which exhibit high solubility in a wide range of functional fluids. These compounds, on account of their high solubility in a wide range of solvents, offer tremendous advantages as adhesion promoters in both aqueous and solvent developable photoresist films. They also obviate the necessity for a persulfate etch to remove the photoresist, a process which destroys deposited copper.
It is an object of this invention, therefore, to provide improved photoresist formulations containing an adhesion promoter which also stabilize the photoresist formulations. The preferred adhesion promoter exhibits the following desired properties: (1) it is capable of easy removal from the metal surface during the development step; (2) it has a sufficiently high boiling point, greater than about 200.degree. C.; (3) it is stable in the coating for a period of one year or longer under ambient conditions; (4) it is stable at elevated temperatures (33.degree.-35.degree. C.) for at least 4-8 weeks; (5) it does not measurably interfere with the photopolymerization reaction; (6) it does not interfere with the plating cycle or leach into the plating bath; and (7) it enables the metal surface to be cleaned with water or weak acid or basic solution without the need for a persulfate etch.